Sky Garden at the Walkie Talkie, London

The Sky Garden is at the very top of the Walkie Talkie building in London (aka 20 Fenchurch Street). It's free to visit although you usually need to book in advance. The view looking down on the River Thames, Tower Bridge, The Tower of London and pretty much everything else in London is excellent. You can also look across at the Shard (which also has a viewing area, but it's expensive to visit). You can walk all the way around and so see in every direction. This is the side facing the River (i.e. South) which is the only side where you can step out onto a balcony (weather permitting). The interior balcony above the bar is in front of the restaurant that occupies the centre of the floor.

BTW it's called the Walkie Talkie as it looks a bit like one (or so they say).